This invention relates to an indexable insert having a polygonal shape in plan and used for cutting machine parts, and more particularly an indexable insert having an improved chip breaker which can break chips discharged during continuous cutting under different cutting conditions into desirable lengths.
FIG. 1A shows a conventional indexable insert having chip breaker grooves. This indexable insert 1 has cutting edges 2 at peripheral edges of the top face (or ridges defined by the top face and the four sides). Chip breaker grooves 3 are formed on the top face along the entire cutting edges 2. Also formed on the top face is a central land 4 from which protrusions 6 extend toward the respective corners 5. As shown in FIG. 1B, each protrusion 6 tapers toward the respective corner, and has its tip cut so as to intersect the bisector of the corner angle at a right angle and define at its front and sides breaker walls 6a that rise obliquely from the breaker groove 3 toward the ridgelines 7 of the protrusion 6.
With this type of indexable insert, the distance from each cutting edge 2 to the terminal end of each breaker groove 3, i.e. to the ridgeline 7 mainly determines the chip curling tendency and chip breakability. Also, this distance determines the permissible feed rate range during normal cutting. That is, the shorter this distance, the higher the chip curling tendency and thus the better the chip breakability and the cutting performance of the insert at a low feed rate. But if this distance is too short, undue curling force tends to act on chips under high feed rate conditions. This increases the possibility of the insert getting clogged with chips and makes cutting unstable. That is, this distance limits the chip disposal range of each insert. Thus, different chip breakers are needed for different cutting conditions.
Various attempts have been proposed to provide inserts which can expand the chip disposal range. One of such inserts is disclosed in examined Japanese utility model publication 1-15442. This insert has, as shown in FIG. 2, protrusions 6 each having arcuate, convex (toward the cutting edges 2) ridgelines 7 intersecting each other on the bisector of the corner angle. With this arrangement, the ridgelines 7 have no points nearest to the straight cutting edges 2, unlike the arrangement of FIG. 1, in which the ridgelines 7 have points 8 nearest to the cutting edges.
The applicant of this invention has proposed a breaker-shaped, triangular indexable insert (in Examined Japanese utility model publication 4-26166) as shown in FIG. 3. This indexable insert 1 has a central land 4, protrusions 6 extending from the land 4 toward the respective corners 5, and ridges 9 lower than the protrusions 6 and provided in the breaker groove 3 between the tips of the protrusions 6 and the respective corners of the cutting edge, and between both sides of the protrusions 6 and the straight portions of the cutting edge. Chips produced during light cutting are disposed of by the breaker walls 9a of the ridges 9. Chips produced during large-depth, high-feed-rate cutting get over the ridge 9 and disposed of by the breaker wall 6a of the protrusion 6.
Chip disposal mechanism is roughly classified into two types. In one type of chip disposal, chip breakers continuously and helically curl long chips by colliding them against the breaker wall. The helical chips thus formed are broken into small pieces having lengths l (shown in FIG. 8) ranging from 20 mm to 50 mm due to their own inertia. This type of chip disposal is hereinafter referred to as continues type chip disposal. This type of chip disposal is mainly used to dispose of chips produced during light cutting such as finish cutting.
In the other type of chip disposal, chip breakers curl chips by colliding them against the breaker wall to break them into small pieces each having a half-curl to one-curl length. Chip disposal of this type is hereinafter referred to as breaking type chip disposal. This type is mainly used for general-purpose cutting other than light cutting, and rough cutting.
Conventional indexable inserts cannot necessarily perform continuous type chip disposal satisfactorily. Thus, breaking type chip disposal was often used even in situations where the continuous type chip disposal is more desirable. Thus, conventional indexable inserts cannot expand the chip disposal range as expected.
For example, in the continuous type chip disposal, the chip breaker shown in FIG. 3 will produce continuous helical chips 40. But depending upon the cutting conditions (depth of cut, feed rate, cutting speed, type of workpiece, etc.), long unbroken chips as shown in FIG. 9 may be produced. Such chips tend to wind around the tool, causing machine trouble. Thus, in the conventional arrangement, in the region where satisfactory chip disposal is impossible with the continuous type chip disposal, the breaking type chip disposal is used by shortening the distance from the cutting edge to the chip breaker. This arrangement however narrows the chip disposal range.
In the continuous chip disposal, no large strain as required in the breaking type disposal has to be applied to chips. Thus, if it is possible to break long chips 40 produced in the continuous type disposal into small pieces as shown in FIG. 8, the distance from the cutting edge to the chip breaker can be increased. This in turn makes it possible to dispose of chips without curling them so markedly not only during light cutting but during general-purpose cutting and rough cutting. The chip disposal range can thus be expanded significantly.
An object of this invention is to answer these requirements by improving the shape of the chip breaker.